Feature Request/Question: QTextCursor and QTextDocumentFragment

[tmanderson]

Currently there are no implementations of QTextCursor and of QTextDocumentFragment. Both types are super helpful when working with `QTextEdit and would be a great addition to nodegui.

Question: Should this be a plugin? Not so sure since QTextEdit API does expose methods returning these useful types, they're just not exposed at the moment.

[tmanderson]

Also, I'd be happy to take up the issue, whatever the answer may be